This invention is generally related to analog signal processing and more particularly to imaging sensors.
Electronic sensors that respond to incident electromagnetic radiation such as light are used in a wide range of applications, from the simple infrared photodetector used in home security systems to complex medical imaging applications. Such sensors provide electrical signals in relation to the energy incident on the sensor. One particular type of sensor is the imaging sensor used for capturing images of objects or scenes from which light energy has been reflected. Such sensors are found in consumer products such as video cameras, scanners, copiers, and, more recently, digital cameras which provide images in a computer-readable format.
A typical imaging sensor is a circuit composed of a number of active semiconductor photocells usually arranged as an array. Examples include the complimentary metal oxide semiconductor (CMOS) active pixel sensor (APS). Each photocell of an imaging sensor has a photodetector and associated control and readout circuitry (including active devices such as transistors). Photogenerated charge in the cell causes a signal to be generated that is a measure of the energy that was incident on the cell. The cell may also feature a saturation limiting circuit which can be used to limit the response of the photocell to the incident energy, and/or an electronic shutter facility which can capture a signal representing the detected incident energy at a specific time.
After the array has been exposed to the object or scene and incident energy has been detected, the signals from the individual cells of the array are collected and may be converted to digital format. This digital image data represents the image that was formed on the sensor array. The digital image data can then be manipulated and displayed using known signal and image processing techniques to yield a desired image size and quality.
One of the problems encountered with imaging sensors is saturation. Saturation occurs when one or more cells in the sensor have been exposed to too much incident energy in view of its dynamic range, i.e., ability to respond faithfully to changes in incident energy. When saturated, the cell output changes too little or not at all in response to more incident energy. An example of the effect of saturation is seen in images as very bright or almost white regions which correspond to saturated cells in the sensor array.
Saturation can be avoided by controlling an electronic shutter circuit in each photocell to limit the total energy detected by the photocell. Some cells provide a signal that represents the instantaneous detected energy and is controlled by the electronic shutter. The signal is integrated over a time interval known as the exposure or integration interval to obtain the total energy. Saturation is avoided in such cells by reducing the integration interval and by controlling the electronic shutter to prevent the signal from reaching a saturation level.
To help determine the proper interval in such imaging systems, a light level test of the scene may be conducted prior to obtaining an image of the scene. During the light level test, the sensor array is exposed to the scene to identify those cells which are receiving strong light and are therefore saturated. Thereafter, the integration intervals for those cells are reduced, and the array is again exposed to the same scene but with the reduced integration times. Lowering the integration interval lowers the amount of incident energy that is detected by the photocell. In a perfect imaging system, the integration times are adjusted so that the cells are allowed to detect the incident energy up to the dynamic range without saturating any cells.
However, the above technique requires the user or the imaging system to guess at the optimum integration interval that avoids saturation yet maximizes dynamic range for the affected photocells. If the integration interval is reduced too much, then dynamic range is reduced as the photocell may still be able to accurately detect more incident energy; if the integration interval is not reduced enough, the photocells can still saturate. Also, taking multiple exposures of the same scene as required by the above technique may prove to be too slow for rapid frame rate applications such as movies.
Therefore, a better technique of preventing saturation in photocells and sensor arrays is desirable that helps maximize dynamic range. Such a technique should also be capable of integration with the manufacture of other electronic circuits, such as those built using standard metal oxide semiconductor (MOS) fabrication processes that are typically used to implement digital functions. In addition, the novel photocell design should be compact to allow improved manufacturing yields in large and dense sensor arrays.